Making sense of life as we know it, in the light of evolution.

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The tenth episode of Planet Earth brings us to the biological communities I think of as “home” — seasonal forests. I grew up in rural Pennsylvania, with second-growth deciduous woodland literally in my back yard, went to college within sight of the Appalachian Mountains, and spent my first “real” job in field ecology surveying understory plant diversity northeast of Pittsburgh. Today, I’m working on the other side of the continent, but now studying some of the most widespread tree species in forests from the Pacific Northwest to the Yukon taiga. I could almost illustrate this entire recap with images from my personal Flickr stream.

We start at what is, arguably, the most seasonal of forests, taiga, where the growing season may last just a month. These snow-covered woods seems marginal, but boreal forests account for one third of the trees on the planet, Sir David Attenborough tells us. The newest comprehensive assessment of tree density worldwide, published last year, found that a median hectare of boreal forest has as many, or slightly more, trees than an average hectare of tropical forest — but it also puts the boreal share of the global tree count at closer to one quarter of all trees, and finds that “tropical moist forest” accounts for a slightly larger share.

Grasslands are defined by a plant community particularly prone to “plant blindness” — it’s easy to see grass as stage-dressing for the dramatic animal life found on the Great Plains, the tundra, or the Tibetan Plateau. But Planet Earth gives grasses their due: “Flooded, burnt, baked, and frozen — grass can withstand it all,” says Sir David Attenborough. The Poaceae is a big, diverse family of plants that grow in some of the harshest conditions on the planet.

Multiple grass lineages independently evolved a new, more efficient form of photosynthesis. As Earth shifted to a cooler and drier climate, with lower atmospheric concentrations of carbon dioxide, starting about 40 million years ago, that may have helped grasslands spread into regions where rainfall and temperature weren’t suitable for forests — and now they cover more than a quarter of the planet’s surface. Some of those grasslands support the biggest assemblages of birds and mammals on the planet, as we see in this episode, which takes full advantage of the openness of its focal community, giving us lots of aerial photography tracking the movements of thousands of caribou, bison, or snow geese.

Most of us in North America know snow geese as winter migrants to lower latitudes, but here we see them on their Arctic breeding ground, establishing and protecting nesting sites — first from other pairs of nesting geese, then from Arctic foxes. Planet Earth frequently follows the nature documentary tactic of employing predator-prey struggles for drama, but the one between the Arctic fox and the snow geese has some twists on the standard script. First, we see the fox trying to gather up so many goslings that she can barely fit them all in her mouth, and we get to cheer as her greed gives the geese time to catch up and run her off. But then we see what she does with the single, sad gosling that doesn’t get away — she brings it to her own clutch of adorable fluffy pups.

Kids! I brought dinner. It’s your favorite — gosling.

Planet Earth is at its best when one adorable fluffy animal is feasting on the flesh of another adorable fluffy animal.

The fox-goose drama is, to my mind, the most interesting predator-prey chase in an episode with a lot of them, most featuring cool camerawork. An aerial camera tracks wolves working together to separate a caribou calf from an immense herd; night-vision footage captures lions taking down an elephant. And on the Tibetan plateau, where pikas keep watchful eyes out for The World’s Most Nonplussed Fox.

Ugh, documentary filmmakers again?

The Tibetan Sand Fox has great pika-stalking moves, but its thick fur squares off its head in a way that makes it look bored, even with a mouthful of pika. Edited into this hunting sequence are shots of watchful pikas, which give the impression that they’re reacting to the fox’s eventual capture of their neighbor.

Oh, god, Bob! He got Bob!

Relatively few of the species we see in this episode are formally recognized as endangered: snow geese, caribou, gray wolves, Arctic foxes, pikas, Tibetan sand foxes, and even most African baboon species are all currently listed as “least concern” by the IUCN. American bison are near-threatened thanks to conservation efforts and semi-domestication — after extensive hunting in the 19th century, there were once as few as 541 in all of North America. Both species of African elephant (the bush elephant and the forest elephant) and lions are listed as Vulnerable, and their numbers are not on the upswing.

The widespread nature of grassland habitats has, perhaps, shielded all these species from the impacts of human activity — though none of them are as numerous, or as widespread, as they once were. And of course they’re all experiencing a human impact that no habitat on Earth can escape: climate change. From the cues governing the migrations of snow geese and caribou to the rainfall that restores the green to the African savannah, the annual cycles of weather that shape life on the grasslands are no longer as certain as they once were. Grasslands arose over a long period of global change, and they may be more resilient to the much faster changes we’ve created than other ecosystems will be — but the great grasslands shown in this episode Planet Earth may look very different in another few decades.

Up to now, though, modern genetic tools have been of limited use for Joshua trees, because no one has assembled the complete DNA sequence of a Joshua tree. Having a “reference genome” would let those of us who study the trees identify specific genes involved in coevolution with yucca moths, compare the evolutionary effects of that pollination mutualism to natural selection exerted by the harsh environments in which the trees grow, and even use genome-scale data to inform Joshua tree conservation planning.

Well, we’ve decided it’s time to do all of that, and we’re asking for help. A team of folks with expertise in Joshua trees’ natural history, Mojave Desert ecology, and genomic data analysis launched the Joshua Tree Genome Project a couple weeks ago, with a crowd-funding campaign on Experiment.com to pay for part of the DNA sequencing we’d need to assemble a reference genome.

We’re approaching 50% of our funding goal, and leading a competition among projects based at undergraduate universities to recruit the most donors, which could win us $2,000 in matching funds — so even if you give as little as $1, you’re providing a big boost to the project. Go check out the Joshua Tree Genome Project website, and then head on over and pledge your support.

Squashes are funny, as fruits go. Even after years of selection for human consumption they have thick, hard rinds, they’re not particularly sweet — in fact, they contain bitter compounds called cucurbitacins — and their seeds don’t separate from their flesh very easily. That all suggests that the wild ancestors of butternuts and pumpkins were dispersed by large mammals, but squashes are native to the Americas, and there haven’t been many large mammals of the sort that would eat them since humans showed up there, back at the end of the last ice age. Of course, those humans since domesticated squashes, which would obviate the need for seed dispersal — and a new genetic study of wild and domesticated squash species provides support for exactly that hypothesis.

[Squashes’] old dispersers were gone and the most likely substitutes were small rodents with diverse diets, who could have chiselled through the fruits and carried the seeds to pastures new. But Kistler found that these same animals are the most likely to be put off by the squashes’ bitter tastes. Compared to larger animals like elephants or rhinos, he found that smaller ones like mice and shrews have far more TAS2R genes, which allow them to taste bitter compounds.

…

Humans can’t stomach cucurbitacins either. People who’ve been unfortunate enough to swallow high levels of these chemicals have come down with a severe diarrhoeal illness called Toxic Squash Syndrome. But perhaps some ancient hunter-gatherers became skilled at finding individual squashes that produced low or tolerable levels. After eating such plants, they would have pooped the seeds out, inadvertently sowing the land with more palatable strains.

I am not personally a great fan of pumpkin pie, particularly since I tried sweet potato last Thanksgiving — but maybe this finding will make for some nice chatter over coffee after dinner. Unless you happen to have an uncle who’s into Pleistocene rewilding, anyway.

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Ecomotion Studios has been working with the Ecological Society of America to produce short animated films about some of the most influential papers of modern ecology — they’re calling it “The Animated Foundations of Ecology.” Here’s the film about Robert Paine’s famous experiment in removing the top predator of tidal pool communities, sea stars, which led to dramatically reduced diversity in the other species that shared the pools.

There’s a handful more, including on one of my favorite classic ecology papers, David Simberloff and EO Wilson’s experimental demonstration of the process by which species colonize new habitats. Go check ’em out!

The new health-related information 23andMe will provide is called carrier status. That relates to whether people have genetic mutations that could lead to a disease in their offspring, presuming the other parent has a mutation in the same gene and the child inherits both mutated genes. There will be information on 36 diseases, including cystic fibrosis, sickle cell anemia and Tay-Sachs.

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Last spring, the journal Current Biology published a report describing something new under the entomological sun: A genus of tiny cave-dwelling insects, dubbed Neotrogla, in which females, not males, have penises.

Or, rather, the females have a thing that they stick inside the males. Once it’s in there, that thing inflates and latches into the male with tiny barbs, binding the couple together in a copulation lasting two to three days, while the thing collects a packet containing sperm and a whole lot of (potentially) nutritious protein. What to call the females’ thing seems to have puzzled even the scientists who described it. In the text of their paper, they call it a gynosome (literally, a “female body”); but in the title, it’s a “female penis.”

This synonymy went from confusing to controversial the moment it hit the popular science press, which almost uniformly chose to go penis-first. “Female insect uses spiky penis to take charge” read the headline in the prestigious journal Nature. “Meet the female insect with giant PENIS whose steamy sex sessions last 70 HOURS,” said the Daily Mirror, caps-locked emphasis sic. Most of the stories, even the Mirror’s, got around to using the word “gynosome” eventually, and many went into more detail about how the organ in question wasn’t really a penis as we know it. LiveSciencenoted it was “a complex organ composed of muscles, ducts, membranes and spikes,” before adding that its size, relative to the body of a Neotrogla female, was “the equivalent of a man who is 5 feet 9 inches (1.75 meters) tall having a penis about 9.8 inches (24.9 centimeters) long.”